Electroless chromium plating



United States Patent ELECTROLESS CHROMIIM PLATING Philip H. Eisenberg, Hicksville, and Douglas 0. Raleigh,

Flushing, N. Y., assignors to Sylvania Electric Products Inc., a corporation of Massachusetts No'Drawing. Application January 26, 1956 Serial No. 561,648

11 Claims. (Cl. 106-1) Our invention relates to both a process in which chromium can be electrolessly deposited on a suitable metallic substrate and to an electroless plating solution used in said process,

The term electroless deposition of chromium as known in the art describes a process wherein. a metallic substrate is inserted in a bath containing in aqueous solution a chromium salt (such as a chloride or sulphate), a hypophosphate salt (such as sodium hypophosphite). and a suitable bufiering (pH controlling) agent (such as sodium acetate). In the ensuing reaction, the. chromium is deposited as a pure metal on the'surface of 'tlie substrate. While the mechanism of this proces'sf'not fully understood, it is believed that the hypophosphit continuously reduces the dissolved metal saltlto the tallic state on the surface of the. substrate. a catalytic action involving, the surface.

It has been found thatthis reaction, whicfiproceeds' I in the absence of an externally applied eIectricfieId, can take place when the pH of the bath is maintained; within the acidic range (as for example a. pI-Lof 46) .or alternatively within the basic range (as for examplefa p I-I of This process is defined as electroless plating or dep sition to differentiate it from the conventional electroplating or electrolytic deposition technique.

The prior art has knowledge of chromium electroless plating solutions which contain, in aqueous media, sodium hypophosphite, sodium citrate, and-a mixtureof chromium halides. A chromium electroless deposition produced through the use of this bath proceeds at a deposition rate of about 0.14 mil per hour. Such a rate is too slow and costly for many otherwise desirable commercial applications of this process.

We have invented a new chromium electroless plating solution which, when employed in an electroless plating operation, overcomes this difiiculty by sharply increasing the chromium deposition rate.

Accordingly it is an object of the present invention to increase the chromium deposition rate in a chromium electroless plating operation.

Another object is to provide a new and improved chromium electroless plating bath of the character indicated.

Still another object is to provide a new and improved chromium electroless plating bath which incorporates as an essential element a chromium depositionrate controlling agent.

Yet another object is to provide an electroless plating method adapted to deposit chromium at a, rate on the order of 2 times higher than that afforded by the aforementioned prior art. 7

These and other objects of our invention will either becxplained or will become apparent hereinafter.

In our invention there is provided a chromium elec troless plating solution containing in aqueous media a hypophosphate salt; at least one soluble chromic salt; a

2,829,059 Patented Apr. 1,. 1958 ice 2: buffering agent; and a deposition rate controlling agent soluble in the plating solution.

When the pH of this-bath is maintained within an acidic or a basic range, and an electroless plating operation is initiated, chromium deposits out of the solution at a rate on the order of 0.3 mil per hour-.-an increase of about 100 percent, as compared to the known art.

We have used the term chromium deposition rate controlling agent to describe the material whose action in solution results in this. extremely rapid, rate of deposition. This agentreduces. the dissolved chromic ions to the chromous state and alsoforms a readily reducible complex with the chromous ions; this complex is. then rapidly reduced to metallic chromium on the surface of the receiving surface through action of the hypophosphite ion. Thus, any, organic or inorganic compound which acts as a combined chromium reducing and chromium complexing agent can. beused, providing that this compound. is. soluble in. the plating. solution and that the resultant complex can be readily reduced to chromium in the manner indicated. Typical compounds of this type include, for example, oxalic-acid and its alkali All. metalsubstrates which can be electroplated with chromiunrca n, be. plated in. the manner set forth in this application. coimectiom it} will'be understood attentional chromiunrl'plating, the passive u us tibeflash coated (i. e. an elecrolytig trike w xtremelythin, layer of any active metarsuen'as' m the, like, before a subseq en el e s ss dn s ib s n. ensue. 1 gomgrnwnmnwnr now, be explained with reference to thejde'tailed examples which follow;

- Example I A steel substrate was immersed, in anaqueous chromium electrolessplating solution having the following compositionz;v

G./l. CI'g-QHzO v 16 CI'Clg-6Hg0 -h 1 Sodium acetate- 10 Sodium hypophosphite 10 Potassium 'binoxalate' 4.5

Example II A brass disc was-flashcoated with nickel and then immersed in an aqueous chromium electroless plating solution having the following composition:

' G./l. CrBr .9H O I 16 CrI .'9H O I "v i I: i 1 Sodium citrate 10 Sodium hypophosphite 10 Sodiumbinoxalate 9.0

l at bath; was maintained. at .a was... within.- the range -90 C. and at a pH falling within the range 8-10. The results were substantially identical with Example I.

v A steel substrate was immersed in the same solution.

The same operating conditionswere established and the Y results were substantially the same.

While we have shown .and pointed out our invention as applied above, it will be apparent "to those skilled 'in the art that many modifications can be made within the scope and sphere of our invention'as defincd'in the claims which'follow.

whatis'claimedisi 2 1. An electroless chromium plating solution'coinposed of the following in aqueous 'solutionra hypophosphite salt having a concentration of aboutjlO grams per liter; a buflering agent; at least one chromic'salt having a eonf centrationof about 17 grams per liter; and a ehromium deposition rate controlling 'agent' soluble in the'fsolution' and characterized by the ability to reduce 'chromic ions to chromous ions and to formareadily:reducible'complex with said chromous ions,' said'agent"being selected from the group consisting of oxalicacid and'its alkali salts, said agent having a concentrationffalling within the approximate range LS-9.0 grams'lpe'r literi' I 2. 'An electroless chromium pla tin" solution composed of the following in aqueous solution: a hypophosphite salt havinga concentration of about Qper liter; a bufiering agent; at least'one chromicsalt having a concentration of about 17 grams 'p'erliter; "and'achromium deposition rate controlling agent soluble in the solution posed in aqueous solution -of a hypophosphite salt having a concentration of about gramsper liter; a buffering agent; at least one chromic salt having a concentration of about -17 gram'sper liter; and a- 'chror'nium deposition rate controlling agent soluble in the solution and characterized by the ability to reduce chromic ions to chromous ions and to form a readily reducible'icomplex with said chromous ion, said agent beingselected from the group consisting of oxalic acid and its alkali salts, said agent having a concentration falling within the approximate range 4.5-9.0 grams per'liter, whereby said substrate is chrome plated through electroless plating action.

4. A method for producing a chromium electroless plating bath which comprises the steps of dissolving a chromium deposition rate controlling agent'into an aqueous solution containing a hypophosphite salt having'a concentration of about 10 grams per liter, a buflering agent, and at least one chromic salt having a concentration of about 17 grams per liter, said rate controlling agent being selected from the group consisting of oxalic acid and its alkali salts, said agent having a concentration falling within the approximate range 4.5-9.0 grams per liter; reacting said rate controlling agent with said chromic ions to reduce said chromic ions to chromous ions and to form a complex with said chromous ions, said complex being easily reducible to metallic chromium in the presence of the hypophosphite ion on a suitably prepared substrate surface.

5. An electroless chromium plating solution composed of the following in aqueous solution a hypophosphite salt having a concentration of about '10 .grams per liter; a buffering agent having an acidic pH value; at least onechromic salt having a concentration of about 17 grams per literiand a deposition rate controlling agent soluble in' the solution and characterized by the ability to reduce chromic ions to chromous ions and to form a complex readily reducible to metallic chromium under electroless catalytic action, said agent being selected from the class consisting of oxalic acid and its alkali salts, said agent having a concentration falling within the approximate range 4.5-9.0 grams per liter and salts of these acids.

6. An electroless chromium plating solution composed of the following in aqueous solution: a hypophosphite salt having a concentration of about 10 grams per liter; a buliering agent having a basic pH value; at least one chromic salt having a concentration of about 17 grams per liter; and a chromium deposition rate controlling agent soluble in the solution and characterized by the ability to reduce chromic ions to chromous ions and to form a complex with said chromous ion readily reducible to metallic chromium under electroless catalytic action, said agent being selected from the class consisting of oxalic acid and its alkali salts, said agent having a con: centration falling within the approximate range 4.59.0 grams per liter. 7

7. A method for chromium plating an active metal sub-' strate which comprises the step of immersing'said sub strate in an electroless chromium plating solution comf posed in aqueous solution of a hypophosphite salt having a concentration of about 10 grams per liter; a buffering agent; at least one'chromic salt having a concentration of about l7 grams per liter; and a chromium deposition rate icontrolling agent soluble in the solution; said'agent reducing the chromic ions to chromous ions and form: ing a readily reducible complex with said chrom'ous'iohs', said complex at the surface of said substrate beingfi're I duced'tofmetallic chromium by the hypophosphitejion, said agent being selected from the group consistingfof oxalic acid and its alkali salts, said agent having 'a con: centration falling within the approximate range 45-90 grams per liter. "j

8. The method as set forth in claim 7, wherein' said active metal substrate is composed of a passive metal'base and an outer coating of an active metal bonded to d base.

9. A method for chromium plating an active iiietal substrate which comprises the steps of dissolving a chro-f mium deposition rate controlling agent into an aqueous solution containing a buffering agent, a hypophosphite salt having a concentration of about 10 grams per liter and at least one chromic salt having a concentration of about 17 grams per liter, said controlling agent reducing the chromic ions to chromous ions and forming a readily reducible complex with said chromous ions, said agent being selected from the group consisting of oxalic acid and its alkali salts, said agent having a concentration falling within the approximate range 4.5-9.0 grams per liter; and inserting said substrate into saidcomplex containing solution, the hypophosphite ion reducing said complex to metallic chromium at the surface of said substrate.

10. The method as set forth in claim 9, wherein the solution is maintained at a pH within the range 4-6, and the temperature of the solution is maintained within the range -90 C. I

11. The method as set forth in claim 9, wherein the solution is maintained at a pH within the range 8-10, and the temperature of the solution is maintained Within the range 75-90 C.

References Cited in the file of this patent UNITED STATES PATENTS Gutzeit Nov. 9,; 1954, 

1. AN ELECTROLESS CHROMIUM PLATING SOLUTION COMPOSED OF THE FOLLOWING IN AQUEOUS SOLUTION: A HYPOPHOSPHITE SALT HAVING A CONCENTRATION OF ABOUT 10 GRAMS PER LITER; A BUFFERING AGENT; AT LEAST ONE CHROMIC SALT HAVING A CONCENTRATION OF ABOUT 17 GRAMS PER LITER; AND A CHROMIUM DEPOSITION RATE CONTROLLING AGENT SOLUBLE IN THE SOLUTION AND CHARACTERIZED BY THE ABILITY TO REDUCE CHROMIC IONS TO CHROMOUS IONS AND TO FORM A READILY REDUCIBLE COMPLEX WITH SAID CHROMOUS IONS, SAID AGENT BEING SELECTED FROM THE GROUP CONSISTING OF OXALIC ACID AND ITS ALKALI SALTS, SAID AGENT HAVING A CONCENTRATION FALLING WITHIN THE APPROXIMATE RANGE 4.5-9.0 GRAMS PER LITER. 